Azo dyes and a method of making a hydrocarbon using an azo dye

ABSTRACT

Azo dyes of the formula ##STR1## where the ring A may be benzofused, 
     n is 0 or 1, and R 1  -R 8  are as defined, 
     as pH-dependent markers for hydrocarbons, hydrocarbons containing the above-mentioned azo dyes, and a method for detecting these azo dyes in hydrocarbons.

This application is a 371 of PCT/EP94/04091 filed Dec. 9, 1994.

BACKGROUND OF THE INVENTION FIELD OF THE INVENTION

The present invention relates to the use of azo dyes of the formula I##STR2## where the ring A may be benzofused,

n is 0 or 1,

R¹ is hydrogen or C₁ -C₁₅ -alkyl which may be interrupted by from 1 to 4ether oxygen atoms,

R² is C₁ -C₁₅ -alkyl which may be interrupted by from 1 to 4 etheroxygen atoms, or a radical of the formula L--NX¹ X², where L is C₂ -C₈-alkylene and X¹ and X² independently of one another are each C₁ -C₆-alkyl or, together with the nitrogen atom linking them, form a5-membered or 6-membered saturated heterocyclic radical which mayfurthermore contain an oxygen atom in the ring,

R³, R⁴, R⁵, R⁶ and R⁷ independently of one another are each hydrogen, C₁-C₁₅ -alkyl or C₁ -C₁₅ -alkoxy and

R⁸ is hydrogen, C₁ -C₁₅ -alkyl, C₁ -C₁₅ -alkoxy, cyano, nitro or aradical of the formula COOX³, where X³ is hydrogen, C₁ -C₁₅ -alkyl whichmay be interrupted by from 1 to 4 ether oxygen atoms, or is a radical ofthe formula L--NX¹ X², where L, X¹ and X² each have the abovementionedmeanings,

as pH-dependent markers for hydrocarbons, hydrocarbons containing theabovementioned azo dyes, a process for detecting these azo dyes inhydrocarbons and novel azo dyes.

DISCUSSION OF THE BACKGROUND

U.S. Pat. No. 5,145,573, U.S. Pat. No. 5,182,372 and EP-A-499 845disclose azo dyes which serve as markers for mineral oils. However, ithas been found that the dyes described there exhibit insufficientdilutability in hydrocarbons.

U.S. Pat. No. 4,009,008 furthermore describes a process for markingmineral oils by means of azo dyes, in which the dye added to the mineraloil is rendered visible by adding to the marked mineral oil an adsorbentwhich binds other colored components of the mineral oil.

It is an object of the present invention to provide novel markers forhydrocarbons. The novel markers should be easily obtainable and readilysoluble in hydrocarbons. Moreover, they should be detectable in a simplemanner. Even very small amounts of marker should be capable of beingrendered visible by a strong color reaction.

SUMMARY OF THE INVENTION

We have found that this object is achieved and that the azo dyes of theformula I which are defined at the outset can advantageously be used aspH-dependent markers for hydrocarbons.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

All alkyl and alkylene radicals occurring in the formula stated here maybe either straight-chain or branched.

If X¹ and X², together with the nitrogen atom linking them, form a5-membered or 6-membered saturated heterocyclic radical which mayfurthermore contain an oxygen atom in the ring, examples of saidradicals which are suitable are pyrrolidinyl, piperidinyl andmorpholinyl.

R¹, R², R³, R⁴, R⁵, R⁶, R⁷, R⁸, X¹, X² and X³ are, for example, methyl,ethyl, propyl, isopropyl, butyl, isobutyl, sec-butyl, pentyl, isopentyl,neopentyl, tert-pentyl, hexyl or 2-methylpentyl.

R¹, R², R³, R⁴, R⁵, R⁶, R⁷, R⁸ and X³ are furthermore, for example,heptyl, octyl, 2-ethylhexyl, isooctyl, nonyl, isononyl, decyl, isodecyl,undecyl, dodecyl, tridecyl, 3,5,5,7-tetramethylnonyl, isotridecyl,tetradecyl or pentadecyl (the above names isooctyl, isononyl, isodecyland isotridecyl are trivial names and originate from the alcoholsobtained by the oxo synthesis; cf. Ullmann's Encyclopedia of IndustrialChemistry, 5th Edition, Vol. A1, pages 290 to 293, and Vol. A 10, pages284 and 285).

R³, R⁴, R⁵, R⁶, R⁷ and R⁸ are furthermore, for example, methoxy, ethoxy,propoxy, isopropoxy, butoxy, isobutoxy, sec-butoxy, pentyloxy,isopentyloxy, neopentyloxy, tert-pentyloxy, hexyloxy, 2-methylpentyloxy,heptyloxy, octyloxy, 2-ethylhexyloxy, isooctyloxy, nonyloxy,isononyloxy, decyloxy, isodecyloxy, undecyloxy, dodecyloxy, tridecyloxy,3,5,5,7-tetramethylnonyloxy, isotridecyloxy, tetradecyloxy orpentadecyloxy.

L is, for example, (CH₂)₂, (CH₂)₃, (CH₂)₄, (CH₂)₅, (CH₂)₆, (CH₂)₇,(CH₂)₈, CH(CH₃)CH₂ or CH(CH₃)CH(CH₃).

R¹, R² and X³ are furthermore, for example, 2-methoxyethyl,2-ethoxyethyl, 2-propoxyethyl, 2-isopropoxyethyl, 2-butoxyethyl, 2- or3-methoxypropyl, 2- or 3-ethoxypropyl, 2- or 3-propoxypropyl, 2- or3-butoxypropyl, 2- or 4-methoxybutyl, 2- or 4-ethoxybutyl, 2- or4-propoxybutyl, 3,6-dioxaheptyl, 3,6-dioxaoctyl, 4,8-dioxanonyl,3,7-dioxaoctyl, 3,7-dioxanonyl, 4,7-dioxaoctyl, 4,7-dioxanonyl, 2- or4-butoxybutyl, 4,8-dioxadecyl, 4,7-dioxaundecyl, 3,6,9-trioxadecyl,3,6,9-trioxaundecyl, 3,6,9-trioxadodecyl, 4,7,10-trioxaundecyl,3,6,9,12-tetraoxatridecyl or 3,6,9,12-tetraoxatetradecyl.

Azo dyes of the formula Ia ##STR3## where R¹ is hydrogen or C₁ -C₁₅-alkyl,

R² and R³ independently of one another are each C₁ -C₁₅ -alkyl and

R⁷ and R⁸ each have the abovementioned meanings,

are preferably used for marking hydrocarbons.

Azo dyes of the formula Ib ##STR4## where R¹ is hydrogen or C₁ -C₁₅-alkyl,

R² is C₁ -C₁₅ -alkyl and

X³ has the abovementioned meanings,

are furthermore preferably used for marking hydrocarbons.

Azo dyes of the formula Ic ##STR5## where R² is C₁ -C₁₅ -alkyl and

R⁵, R⁶, R⁷ and R⁸ each have the abovementioned meanings, are alsopreferably used for marking hydrocarbons.

Azo dyes of the formula Ia, where R¹ and R² independently of one anotherare each C₁ -C₁₃ -alkyl, R³ is methyl, R⁷ is hydrogen and R₈ is aradical of the formula COOX³, where X³ is C₁ -C₁₃ -alkyl, areparticularly preferably used for marking hydrocarbons.

Azo dyes of the formula Ic, where R² is C₁ -C₁₃ -alkyl, R⁵ and R⁷ areeach methyl and R⁶ and R⁸ are each hydrogen, are furthermoreparticularly preferably used for marking hydrocarbons.

Some of the dyes of the formula I are known and are described, forexample, in GB-A-953 719, U.S. Pat. No. 3,218,309 or U.S. Pat. No.4,037,007.

For the purposes of the present invention, pH-dependent markers are tobe understood as meaning those azo dyes of the formula I which, underthe action of a protic acid, in the presence or absence of a halide ofthe metals zinc, aluminum or tin, give a color reaction, ie. a colorchange, accompanied by a deepening of color.

For the purposes of the present invention, marking is to be understoodas meaning the addition of the azo dyes of the formula I to hydrocarbonsin concentrations such that the hydrocarbons appear to the human eye tohave either no color at all or only a slight color, but the dyes of theformula I are readily detectable in a clearly visible manner by thedetection methods described in detail here.

The present invention furthermore relates to hydrocarbons containing oneor more of the azo dyes of the formula I.

For the purposes of the present invention, hydrocarbons are to beunderstood as meaning aliphatic or aromatic hydrocarbons which are inthe liquid state under standard conditions of temperature and pressure.These are in particular mineral oils, for example fuels, such asgasoline, kerosine or diesel oil, or oils, such as heating oil or motoroil.

The azo dyes of the formula I are suitable in particular for markingmineral oils which have to be identified, for example for tax reasons.In order to keep the costs of the identification low, it is desirable touse very small amounts of markers.

The azo dyes of the formula I, either in the absence of solvent or inthe form of solutions, are used for marking hydrocarbons. Suitablesolvents are organic solvents. Aromatic hydrocarbons, such as toluene,xylene, dodecylbenzene, diisopropylnaphthalene or a mixture of higheraromatics which is commercially available under the name Shellsol® AB(from Shell), are preferably used. In order to avoid a high viscosity ofthe resulting solutions, in general a concentration of azo dye I of from20 to 80% by weight, based on the solution, is chosen.

Further cosolvents, for examples alcohols, such as methanol, ethanol,propanol, isopropanol, butanol, isobutanol, pentanol, hexanol, heptanol,octanol, 2-ethylhexanol or cyclohexanol, glycols, such as butylethyleneglycol or methylpropylene glycol, amines, such as triethylamine,diisooctylamine, dicyclohexylamine, aniline, N-methylaniline,N,N-dimethylaniline, toluidine or xylidine, alkanolamines, such as3-(2-methoxyethoxy)propylamine, o-cresol, m-cresol or p-cresol, ketones,such as diethylketone or cyclohexanone, lactams, such as γ-butyrolactam,carbonates, such as ethylene carbonate or propylene carbonate, phenols,such as tert-butylphenol or nonylphenol, esters, such as methylphthalate, ethyl phthalate, 2-ethylhexyl phthalate, ethyl acetate, butylacetate or cyclohexyl acetate, amides, such as N,N-dimethylformamide,N,N-diethylacetamide or N-methylpyrrolidone, or mixtures thereof, may beused for improving the solubility.

By means of the azo dyes of the formula I which are to be used accordingto the invention, it is possible in a very simple manner to detectmarked hydrocarbons, even when the marking substances are present onlyin a concentration of about 10 ppm or less.

In some cases, it is also advantageous to use mixtures of dyes of theformula I with one another as marking substances.

The presence of the azo dyes of the formula I, used as markers, inhydrocarbons is advantageously detected if the hydrocarbon is treatedwith an aqueous alcoholic or alcoholic medium which contains a proticacid and, if required, a halide of the metals zinc, aluminum or tin.When aqueous alcoholic media are used, the weight ratio of water toalcohol is from 0.5:1 to 4:1, preferably about 1:1.

When the protic acid and, if required, the metal halide are added to themarked hydrocarbon, a clearly visible color reaction results and, in thecase of the use of an aqueous alcoholic medium, the azo dye I passesover into the aqueous alcoholic phase.

Examples of suitable alcohols are ethanol, propanol, isopropanol,1-methoxypropan-2-ol, ethylene glycol and 1,2- and 1,3-propylene glycol.The use of ethanol is preferred.

Suitable protic acids for the novel process are in particular strongacids, ie. protic acids whose pKa value is ≦3.5. Examples of suitableacids of this type are inorganic or organic acids, such as perchloricacid, hydriodic acid, hydrochloric acid, hydrobromic acid, hydrofluoricacid, sulfuric acid, nitric acid, phosphoric acid, benzenesulfonic acid,toluenesulfonic acid, naphthalenesulfonic acid, methanesulfonic acid,oxalic acid, maleic acid, chloroacetic acid, dichloroacetic acid andbromoacetic acid. In some cases, it may be advantageous to buffer theseacids, for example by adding acetic acid.

In addition to o- or p-toluenesulfonic acid, inorganic acids areparticularly noteworthy, hydrochloric acid or sulfuric acid beingespecially important.

Suitable halides of the metals zinc, aluminum or tin are, for example,zinc chloride, zinc bromide, aluminum chloride, aluminum bromide or tintetrachloride. Zinc chloride is particularly noteworthy.

As a rule, it is sufficient to extract an amount of from about 10 to 50ml of the hydrocarbon marked according to the invention with from 10 to50 ml of an aqueous alcoholic or alcoholic solution of a protic acid,with or without the addition of the metal halide, in order to obtainthis color reaction. It is also possible to use an aqueous alcoholicsolution of the metal halide alone, since this is likewise acidic.

The concentration of the protic acid in the aqueous alcoholic oralcoholic solution is as a rule from 5 to 50, preferably from 10 to 30,% by weight. The concentration of metal halide is in general from 0 to50, preferably from 5 to 20, % by weight, based on each case on theweight of the solution.

The present invention furthermore relates to azo dyes of the formula II##STR6## where n is 0 or 1

and, when n is 0,

Z¹ is C₁ -C₁₅ -alkyl which may be interrupted by from 1 to 4 etheroxygen atoms, or is a radical of the formula L--NX¹ X², where L is C₂-C₈ -alkylene and X¹ and X² independently of one another are each C₁ -C₆-alkyl or, together with the nitrogen atom linking them, form a5-membered or 6-membered saturated heterocyclic radical which mayfurthermore contain an oxygen atom in the ring,

R⁷ is hydrogen, C₁ -C₁₅ -alkyl or C₁ -C₁₅ -alkoxy and

Z² is a radical of the formula COOZ³, where Z³ is C₁ -C₁₅ -alkyl whichmay be interrupted by from 1 to 4 ether oxygen atoms, or is a radical ofthe formula L--NX¹ X², where L, X¹ and X² each have the abovementionedmeanings,

with the proviso that, when Z¹ is L--NX¹ X², the sum of the carbon atomspresent in the radicals X¹, X² and Z³ is at least 8,

or, when n is 1,

Z¹ is a radical of the formula L--NX¹ X², where L, X¹ and X² each havethe abovementioned meanings, and

R⁵, R⁶, R⁷ and Z² are each hydrogen, C₁ -C₁₅ -alkyl or C₁ -C₁₅ -alkoxy,

with the proviso that, in each of the pairs of radicals R⁵ /R⁶ and R⁷/Z², one radical is not hydrogen.

The present invention furthermore relates to azo dyes of the formula III##STR7## where Q¹ is hydrogen or C₁ -C₁₅ -alkyl,

Q² is C₁ -C₁₅ -alkyl,

Q³ is C₁ -C₁₅ -alkyl or C₁ -C₁₅ -alkoxy,

Q⁴ is C₁ -C₁₅ -alkyl or C₁ -C₁₅ -alkoxy and

Q⁵ is C₁ -C₁₅ -alkyl which may be interrupted by from 1 to 4 etheroxygen atoms, or is a radical of the formula L--NX¹ X², where L is C₂-C₈ -alkylene and X¹ and X² independently of one another are each C₁ -C₆-alkyl or, together with the nitrogen atom linking them, form a5-membered or 6-membered saturated heterocyclic radical which mayfurthermore contain an oxygen atom in the ring,

with the proviso that the sum of the carbon atoms present in theradicals Q¹, Q² and Q⁵ is at least 8.

Azo dyes of the formula II where n is 0 are preferred.

Azo dyes of the formula II, where n is 0, Z¹ is C₁ -C₁₃ -alkyl, R⁷ ishydrogen and Z² is COOZ³, where Z³ has the abovementioned meanings, areparticularly preferred.

Azo dyes of the formula III, where Q¹, Q² and Q³ independently of oneanother are each C₁ -C₁₅ -alkyl and Q⁴ is hydrogen, are also preferred.

Azo dyes of the formula III, where Q¹ and Q² independently of oneanother are each C₁ -C₁₃ -alkyl, Q³ is methyl, Q⁴ is hydrogen and Q⁵ isC₁ -C₁₃ -alkyl, are also particularly preferred.

The novel dyes of the formulae II and III can be obtained by methodsknown per se.

For the preparation of the dyes of the formula II, for example, an amineof the formula IVa or IVb ##STR8## where R⁵, R⁶, R⁷ and Z² each have theabovementioned meanings, can be diazotized in a manner known per se andthe product coupled with a coupling component of the formula V ##STR9##where Z¹ has the abovementioned meanings.

The amines of the formula IVb in turn are obtainable by diazotizing anamine of the formula IVa and then coupling the product with an anilineof the formula IVc ##STR10## where R⁵ and R⁶ each have theabovementioned meanings.

For the preparation of the dyes of the formula III, for example, anamine of the formula VI ##STR11## where Q⁴ and Q⁵ each have theabovementioned meanings, can be diazotized in a manner known per se andthe product coupled with a coupling component of the formula VII##STR12## where Q¹, Q² and Q³ each have the abovementioned meanings.

The novel azo dyes of the formulae II and III have good solubility inorganic solvents and, as stated above, can advantageously be used aspH-dependent markers for hydrocarbons.

The examples which follow illustrate the invention.

A) Preparation

EXAMPLE 1

107 g of 4-(3'-methylphenylazo)-3-methylaniline hydrochloride and 0.5 gof a anionic surfactant were suspended in a mixture of 50 ml of waterand 30 ml of 5N hydrochloric acid at room temperature. 50 g of ice and20 ml of toluene were added, after which a concentrated aqueous solutionof 6.9 g of sodium nitrite was introduced. The diazotization wascompleted in the course of 2 hours at from 5° to 10° C., after which theexcess nitrite was removed using amidosulfonic acid. In order todissolve the diazonium salt, a solution of 29.5 g of1-(3-diethylaminopropyl)aminonaphthalene in 35 ml of toluene was thenadded dropwise at from 10° to 15° C. Thereafter, the pH of the reactionmixture was increased to about 4 with 2.5N sodium acetate solution, andthe coupling was complete after 30 minutes. After the pH had beenincreased to about 8 with 10N sodium hydroxide solution, phaseseparation occurred. The organic phase was then washed salt-free at 60°C. by extracting several times with water. Distilling off the toluenegave 50 g of an oil-like dye of the formula ##STR13## which readilydissolves in aromatics to give a red solution. λ_(max) (toluene): 522nm.

EXAMPLE 2

35.2 g of a 58% strength by weight aqueous solution of2-dimethylaminoethyl anthranilate were added dropwise to a mixture of 33ml of 10N hydrochloric acid and 10 ml of glacial acetic acid at from 5°to 10° C. A concentrated aqueous solution of 6.9 g of sodium nitrite wasthen added at from 10° to 15° C., the pH being kept below 0.5. Thediazotization was complete in the course of 30 minutes at from 10° to15° C., after which the excess nitrite was removed using amidosulfonicacid. 1 g of an anionic surfactant was added, after which 26.2 g of1-(2-ethylhexylamino)naphthalene, dissolved in 30 ml of glacial aceticacid, were added dropwise to the solution of the diazonium salt at from10° to 15° C. in the course of 30 minutes. Thereafter, the batch wasdiluted with 100 ml of water, and the coupling was complete after 2hours. After 100 ml of toluene had been added and the pH increased toabout 7 with 10N sodium hydroxide solution, phase separation occurred.The organic phase was then washed salt-free at 60° C. by extractingseveral times with water. Distilling off the toluene gave 48 g of anoil-like dye of the formula ##STR14## which readily dissolved inaromatics to give an orange solution. λ_(max) (toluene): 448 nm.

B) Use

General method

A 40% strength by weight solution of the marker in a commercial mixtureof higher aromatics (Shellsol® AB from Shell) is added to commercialdiesel fuel.

The amount of marker added is 10 ppm.

a) 50 ml of an acidic test solution consisting of 45 g of water, 45 g ofethanol, 5 g of p-toluenesulfonic acid and 5 g of zinc chloride areadded to 50 ml of marked diesel fuel in a separating funnel. The mixtureis vigorously shaken for about 10 seconds. After the phases haveseparated again, the test solution phase is found to have an intensecolor. The aqueous phase can be measured photometrically against asolution of known concentration.

b) 20 ml of marked diesel fuel are vigorously shaken with 20 ml ofreagent solution (10% strength by weight zinc chloride solution in awater/ethanol mixture (60:40 v/v), pH brought to 2 by adding 85%strength by weight acetic acid). The lower, aqueous phase acquires aclearly detectable color. The aqueous phase can be measuredphotometrically against a solution of known concentration.

The dyes shown in the tables below were detected by method a). They canalso be detected just as successfully by method b).

In the tables below, the λ_(max) values indicated by *) were eachmeasured in toluene, and the λ_(max) values indicated by **) were eachmeasured in water:ethanol:p-toluenesulfonic acid:zinc chloride 45:45:5:5(w/w/w/w).

                  TABLE 1                                                         ______________________________________                                         ##STR15##                                                                    Ex.                              λ.sub.max *.sup.)                                                            λ.sub.max **.sup.)              No.  K.sup.1      K.sup.2         nm!   nm!                                   ______________________________________                                        3    CH.sub.3     C.sub.2 H.sub.5                                                                              447   539                                    4    CH.sub.2 CH(C.sub.2 H.sub.5)C.sub.4 H.sub.9                                                C.sub.2 H.sub.5                                                                              444   540                                    5    CH.sub.2 CH(C.sub.2 H.sub.5)C.sub.4 H.sub.9                                                CH.sub.2 CH(C.sub.2 H.sub.5)C.sub.4 H.sub.9                                                  445   543                                    6    CH.sub.3     (CH.sub.2).sub.3 O(CH.sub.2).sub.2 OCH.sub.3                                                 454   540                                    7    CH.sub.2 CH(C.sub.2 H.sub.5)C.sub.4 H.sub.9                                                 ##STR16##     454   554                                    8    CH.sub.2 CH(C.sub.2 H.sub.5)C.sub.4 H.sub.9                                                (CH.sub.2).sub.3N(C.sub.2 H.sub.5).sub.2                                                     458   544                                    9    (CH.sub.2).sub.2N(CH.sub.3).sub.2                                                          C.sub.2 H.sub.5                                                                              447   540                                    10   CH.sub.3     CH.sub.2 CH(C.sub.2 H.sub.5)C.sub.4 H.sub.9                                                  446   542                                    11   CH.sub.2 CH(C.sub.2 H.sub.5)C.sub.4 H.sub.9                                                (CH.sub.2).sub.2N(iso-C.sub.3 H.sub.7).sub.2                                                 450   554                                    12   (CH.sub.2).sub.2N(CH.sub.3).sub.2                                                          iso-C.sub.13 H.sub.27                                                                        448   544                                    13   CH.sub.2 CH(C.sub.2 H.sub.5)C.sub.4 H.sub.9                                                (CH.sub.2).sub.2N(C.sub.4 H.sub.9).sub.2                                                     448   552                                    14   CH.sub.2 CH(C.sub.2 H.sub.5)C.sub.4 H.sub.9                                                 ##STR17##     460   546                                    ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                         ##STR18##                                                                    Ex.                              λ.sub.max *.sup.)                                                             λ.sub.max **.sup.)             No.   K.sup.1      K.sup.2 K.sup.3                                                                              nm!    nm!                                  ______________________________________                                        15    CH.sub.3     C.sub.2 H.sub.5                                                                       C.sub.2 H.sub.5                                                                     425    519                                   16    CH.sub.2 CH(C.sub.2 H.sub.5)C.sub.4 H.sub.9                                                C.sub.2 H.sub.5                                                                       C.sub.2 H.sub.5                                                                     422    518                                   17    (CH.sub.2).sub.2N(CH.sub.3).sub.2                                                          C.sub.2 H.sub.5                                                                       C.sub.2 H.sub.5                                                                     424    520                                   ______________________________________                                    

                  TABLE 3                                                         ______________________________________                                         ##STR19##                                                                    Ex.                         λ.sub.max *.sup.)                                                              λ.sub.max **.sup.)                 No.  K                       nm!     nm!                                      ______________________________________                                        18                                                                                  ##STR20##             513     582                                       19   (CH.sub.2).sub.3N(C.sub.2 H.sub.5).sub.2                                                             512     592                                       20   C.sub.2 H.sub.5        507     579                                       21   CH.sub.2 CH(C.sub.2 H.sub.5)C.sub.4 H.sub.9                                                          504     582                                       22   mixture (1:1) of CH.sub.2 CH(C.sub.2 H.sub.5)C.sub.4 H.sub.9                                         506     582                                            isotridecyl                                                              23   (CH.sub.2).sub.3 O(CH.sub.2).sub.2 OCH.sub.3                                                         514     582                                       24   (CH.sub.2).sub.3 O(CH.sub.2).sub.2 OC.sub.4 H.sub.9                                                  515     582                                       25   mixture (1:1) of (CH.sub.2).sub.3 O(CH.sub.2).sub.2 OCH.sub.3                                        514     578                                            and (CH.sub.2).sub.3 O(CH.sub.2).sub.2 OC.sub.4 H.sub.9                  26   (CH.sub.2).sub.3 O(CH.sub.2).sub.2 O(CH.sub.2).sub.2 OCH.sub.3                                       514     580                                       27   (CH.sub.2).sub.2N(C.sub.4 H.sub.9).sub.2                                                             509     592                                       28                                                                                  ##STR21##             520     584                                       29   iso-C.sub.13 H.sub.27  503     580                                       ______________________________________                                    

                  TABLE 4                                                         ______________________________________                                         ##STR22##                                                                    Ex.                                λ.sub.max *.sup.)                                                            λ.sub.max **.sup.)            No.  K.sup.1 K.sup.2 K.sup.3        nm!   nm!                                 ______________________________________                                        30   H       OCH.sub.3                                                                             C.sub.2 H.sub.5                                                                             447   565                                  31   CH.sub.3 O                                                                            H       C.sub.2 H.sub.5                                                                             439   582                                  32   CH.sub.3 O                                                                            H       CH.sub.2 CH(C.sub.2 H.sub.5)C.sub.4 H.sub.9                                                 440   588                                  33   CH.sub.3 O                                                                            H       (CH.sub.2).sub.3 O(CH.sub.2).sub.2 OCH.sub.3                                                445   585                                  34   CH.sub.3 O                                                                            H       mixture (1:1) of                                                                            444   584                                                       (CH.sub.2).sub.3 O(CH.sub.2).sub.2 OCH.sub.3 and                              (CH.sub.2).sub.3 O(CH.sub.2).sub.2 OC.sub.4 H.sub.9      35   C.sub.2 H.sub.5 O                                                                     H       (CH.sub.2).sub.3 O(CH.sub.2).sub.2 OCH.sub.3                                                445   587                                  36   C.sub.2 H.sub.5 O                                                                     H       (CH.sub.2).sub.3 O(CH.sub.2).sub.2 OC.sub.4 H.sub.9                                         444   588                                  ______________________________________                                    

                                      TABLE 5                                     __________________________________________________________________________     ##STR23##                                                                    Ex.                         λ.sub.max *.sup.)                                                            λ.sub.max **.sup.)                   No.                                                                              K.sup.1   K.sup.2                                                                          K.sup.3                                                                           K.sup.4                                                                           K.sup.5                                                                            nm!   nm!                                        __________________________________________________________________________    37                                                                                ##STR24##                                                                              H  CH.sub.3                                                                          C.sub.2 H.sub.5                                                                   C.sub.2 H.sub.5                                                                   482   558                                         38 CH.sub.3 O                                                                              H  H   C.sub.2 H.sub.5                                                                   C.sub.2 H.sub.5                                                                   418   555                                         __________________________________________________________________________

We claim:
 1. A method of marking a hydrocarbon, comprising adding to a hydrocarbon an azo dye of the formula Ib ##STR25## where R¹ is hydrogen or C₁ -C₁₅ -alkyl,R² is C₁ -C₁₅ -alkyl and X³ is hydrogen, C₁ -C₁₅ -alkyl which is uninterrupted or interrupted by from 1 to 4 ether oxygen atoms or is a radical of the formula LNX¹ X², where L is C₂ -C₈ -alkylene and X¹ and X² independently of one another are each C₁ -C₆ -alkyl or, together with the nitrogen atom linking them, form a 5-membered or 6-membered saturated heterocyclic radical which does or does not furthermore contain an oxygen atom in the ring.
 2. A hydrocarbon containing one or more azo dyes, as pH-dependent markers, of the formula Ib ##STR26## where R¹ is hydrogen or C₁ -C₁₅ -alkyl,R² is C₁ -C₁₅ -alkyl and X³ is hydrogen, C₁ -C₁₅ -alkyl which is uninterrupted or interrupted by from 1 to 4 ether oxygen atoms or is a radical of the formula LNX¹ X², where L is C₂ -C⁸ -alkylene and X¹ and X² independently of one another are each C₁ -C₆ -alkyl or, together with the nitrogen atoms linking them, form a 5-membered or 6-membered saturated heterocyclic radical which does or does not furthermore contain an oxygen atom in the ring. 